If he thinks the officers in the DPD would follow an order to keep the feds out of the city/county he's only fooling himself and setting himself up to be arrested. They might have to follow the order not to comply with detention orders because federal laws only apply to federal officers, but local officials have no control over federal officers in the performance of their duties.
Feds could sit on the courthouse or jail waiting for Denver to release an illegal alien on bond to arrest them for a federal crime and there's not a single thing that Johnson, or Polis for that matter, could do about it. Not a single thing they could do about interfering with a federal arrest warrant being executed anywhere in the city or State either.